The present invention relates to a controller of a PWM inverter, and more particularly to a digital PWM control circuit for converting a reference signal to a PWM signal having a pulse width corresponding to an output voltage of an inverter, thereby controlling the output voltage of the inverter.
Bipolar transistor inverters performing PWM-control have been widely used as economical variable-speed controllers for AC motors. However, according to its operation principle, a PWM inverter inevitably generates noise of the modulation frequency. Since a conventional high-power transistor inverter can perform pulse width modulation (PWM) at a modulation frequency of several kilohertz at most, it generates considerable audible noise, and its general application is thus limited. Therefore, a low-operation-noise PWM inverter has been demanded.
If a power FET (Field-Effect Transistor), which can operate at a speed higher than the operation speed of a bipolar transistor, or an IGT (Insulated Gate Transistor) is used for the PWM control, a modulation frequency of several tens kilohertz (non-audible frequency) can be achieved. When such a high-speed switching element is used to constitute an inverter, a high-speed PWM control circuit is required for the inverter.
In a conventional PWM control, a triangular wave carrier is compared with a reference signal wave to provide a PWM signal. However, according to such a conventional PWM control, it is difficult to actually operate with a triangular wave of several tens kilohertz, and its operation speed cannot be made so high. In addition, if the average level of the triangular wave carrier and that of the reference signal wave do not coincide with each other, a DC component is involved in the PWM control output. In this case, when multiphase PWM control is to be performed, unbalances occur among respective phases. For this reason, according to a conventional PWM control, a fine level adjustment is required for each phase of the multiphase PWM control.